Imagine, for a moment, that you are the manager of a professional bat-and-ball club. The game is about to commence and your starting nine are all ready to go. But while you know who you'll be sending out to the field, you have yet to determine how your defensive lineup will look.
All the spots have been filled but two: second base and third base. The players you're considering—let's call them Tony Stark and Bruce Wayne—are both equally comfortable at the keystone and the hot corner. Stark is clearly the superior defender (we'll say he's a +10-run true-talent fielder in whatever fielding metric you prefer, and put Wayne at -10) but the team's aggregate defense is the same no matter which you put where.
The first man to step to the plate for the opposing team is a left-handed pull hitter in the mold of David Ortiz, meaning that the ball is almost assuredly going to the right side of the field. Forgetting about that crazy infield shift teams now put on for players like him (or assuming that the hypothetical hitter isn't quite enough of a pull guy to warrant it), which fielders do you want where?
The answer, to me, seems pretty clear: you want Stark at second base, because the odds of the ball being hit there are higher than that of the ball coming to third. You'd feel much more secure with him fielding the ball than with it going "pastadiving
Jeter Wayne," so you put the better player where he'll have more opportunities to do his thing. It's the same reason Shane Victorino hits higher in the order than Roy Halladay.
But the other team's lineup consists of more than just Hafners and Thomes. The second batter of the game is a very different kind of hitter: a right-handed pull guy, like Jose Bautista. Suddenly, the ball is more likely to come to the left side of the field, and things at the hot corner will start to heat up. Would you rather have a relative hole at second base for Ortiz, or at third base for Bautista?
The answer: that's a false dichotomy. The best strategy would be to start Stark at second base, then move him to third when Bautista comes to bat.
I've never seen or heard of a team doing this before, but assuming two players wouldn't feel too out of place if they swapped positions (to be fair, there probably aren't many teams who could say that right now), why shouldn't the manager try to maximize the efficiency of the defense by putting the better glovesman where the action is for each batter?
Ladies and gentlemen, I give you the defensive platoon.
As another example that may seem more realistic, imagine if Stark and Wayne were instead corner outfielders; Stark channels late-90's Andruw Jones in the field while Wayne running for the ball conjures up images of Adam Dunn and Manny Ramirez competing in a three-legged race. When Ortiz is up, surely you'd rather put Stark in right field. But when Bautista comes to the plate, you wouldn't want Wayne manning his left field power alley.
Maximizing defensive efficiency could be based partly on the pitcher, too. If Stark and Wayne are now competing for playing time at shortstop and center field and 50% flyball rate man Colby Lewis is on the mound, you'd probably want to put Stark in center—especially if you're playing in cavernous Petco Park. If instead you're facing an extreme wormburner like Jake Westbrook or Justin Masterson, you'd want the superior glove at shortstop. Even more so if you're playing in Arlington.
This is all based on the assumption that these players are not just able to play many positions, but able to do so at similar levels. However, even if that's not the case, the system could still work. If Stark is a natural third baseman, he may be only a +5 guy at second base. But if Ortiz is up, moving him to second still might be a smart play—there's no real way to evaluate this without more specific data for each player involved, but the marginal value of the switch under the circumstances could outweigh the fact that the team's overall defense would get worse.
Still, the notion of mid-game position swapping without substitutions seems wrong. A player's position is as much a part of his identity as his height and handedness. Except occasionally in the minors or with extensive work in spring training, you don't think of it as something that can be changed. I doubt this idea would be as controversial among traditionalists as, say, DIPS theory or the inadvisability of sacrifice bunts, but it's not the way things are done. This weekend, for example, Indians manager Manny Acta said it would have been "unfair" to put Kosuke Fukudome in left field so superior athlete Ezequiel Carrera could play center because Fukudome has never played in that specific third of the big green in his MLB career. If managers are uncomfortable with center fielders moving to left, I'm guessing they wouldn't be too receptive to moving them to third base.
Nor is this a particularly practical approach—at least, not now. Even if all 30 MLB managers were to read this, find it convincing, and decide they weren't worried about potential backlash from fans, the vast majority wouldn't have the means to do anything about it. For most players (or at least the ones good enough to be in the starting lineup on a regular basis), their positions have been their specialties for years, and the odds of having two good players whose versatility lines up on the same team aren't very high.
Perhaps this means versatility could be the next undervalued skillset on the baseball market. Maybe it would have been in, for example, the Blue Jays' best interest to continue to work on Brett Lawrie's defense at second base too instead of converting him whole-hog to a third baseman, and the Red Sox would have much more flexibility if Dustin Pedroia could still play shortstop anywhere close to as well as he can second base.
That being said, there are already a number of versatile players who aren't just benchwarmers or utility men. Ben Zobrist is the obvious current face of the group, but a number of other high-profile players have gotten around the diamond in their careers as well. Alex Rodriguez moved from shortstop to third when he joined the Yankees. Albert Pujols went from third to left before landing at first. Chone Figgins, Brandon Inge, Pablo Sandoval, Ryan Braun, and Kevin Youkilis are just a few examples of players who have moved around in their careers (not to mention Darin Erstad, who won Gold Gloves at two different positions). That doesn't mean they'd all be able to move back to their old places tomorrow, but it shows that they found versatility when their teams needed them to move.
It probably wouldn't be worth teaching veterans to play new positions just to be able to implement an adaptive defense now, but if teams plan ahead it could work in the future. And I'd love to see a team that already has a versatile roster (say, the Rays) try out a defensive platoon—even if it improved their odds of winning each game by only 1%, that would mean a difference of nearly two wins a year.
This is essentially the defensive equivalent of lineup optimization. Fielding isn't as important as hitting, so this definitely wouldn't make or break a team. But in baseball, any opportunity to gain a strategic advantage is an opportunity worth taking, and it seems to me that an open-minded team with a versatile roster could use defense platoons to get in a better position to win.